Caprolactam-Based Composition, Process for Manufacturing an Impermeable Element, and Tank

ABSTRACT

The present invention relates to a composition that can be used to manufacture an impermeable, sealing, tight envelope, to a process for manufacturing an impermeable envelope, and to a tank. The composition of the invention comprises in % by weight relative to the total weight of the composition: from 70 to 90% of a monomer (I); from 0.1 to 1% of an activator (II), in which R is chosen from the group comprising C n H 2n+2 , n being an integer chosen from 1 to 10; —OH; —OC n H 2n+2 , n being an integer chosen from 1 to 10; and —NHR′ where R′ is either C n H 2n+2 , n being an integer chosen from 1 to 10, or an amine functional group; from 2 to 6% of a catalyst (III), in which X is chosen from the group comprising MgBr, MgI, Li and Na; and from 10 to 20% of an additive (IV), with: 
     
       
         
         
             
             
         
       
     
     This composition can be used, for example, to manufacture elements that are impermeable to fluids, for example impermeable envelopes, for example that can be used in the manufacture of type IV tanks or hydraulic accumulators.

This application is a Divisional application of U.S. Ser. No.12/094,440, filed May 21, 2008, which is a National Stage application ofInternational Application No. PCT/EP2006/068816 filed Nov. 23, 2006, theentire contents of which is hereby incorporated herein by reference.This application also claims the benefit under 35 U.S.C. §119 of FrenchPatent Application No. 05 53585, filed Nov. 24, 2005, the entirecontents of which is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a composition that can be used tomanufacture an impermeable element (sealing element) made ofpolycaprolactam or polyamide PA-6, for example to manufacture envelopesthat are impermeable, tight, (sealing envelope) to fluids, in particularto pressurized gases.

The present invention also relates to a process for manufacturing animpermeable, tight, envelope (sealing envelope) and to a tank, reservoircapable of being obtained (obtainable) by this process.

The impermeable, tight, sealing envelopes of the present invention maybe used, for example, for manufacturing type IV tanks or hydraulicaccumulators. Composite type IV tanks are tanks in which the pressure ofthe gases stored is generally from 10⁶ to 10⁸ Pa. Their structure istherefore designed, on the one hand, to be impermeable to the gasesstored and, on the other hand, to withstand the storage pressures ofthese gases: it comprises an inner envelope or bladder that isimpermeable to the gas, also known as a liner, and an outer reinforcingstructure usually composed of carbon fibres and thermosetting resin.

The present invention finds, for example, applications in themanufacture of low-temperature fuel cells (for example, proton exchangemembrane fuel cells or PEMFCs).

In the description below, the references between brackets ([ ]) refer tothe list of references given after the examples.

STATE OF THE ART

Type IV tanks were developed in the 1990s, first for storing natural gaswith polyethylene liners and, more recently, mainly from 1997 onwards,for storing hydrogen.

The thermoplastic liners currently used are for the most part composedof polyethylenes (PEs) which are usually of high density (HDPEs) andsometimes crosslinked (XHDPEs). Other thermoplastics of polyamide (PA)type (usually known as “nylon”™) of PA-6, PA-12 or PA-11 type are alsoused as they have gas barrier properties that are intrinsically betterthan polyethylene. Finally, other types of more technical thermoplasticsmay be used because they have good gas barrier properties, such aspolyvinyldene difluoride (PVDF) or multilayer solutions with a barrierlayer made of an ethylene/vinyl alcohol copolymer (EVOH). Documents [1]and [2] describe such thermoplastics.

Most of the time, these liners are obtained by rotomoulding or extrusionand/or blow moulding of the thermoplastic in the melt state. Thus, indocument [3], it is mentioned that the thermoplastic liner is obtainedby extrusion-blow moulding or rotomoulding, preferably using a high ormedium density polyethylene. In document [4], impermeable liners made ofpolyethylene, polypropylene or polyamide are obtained by rotomoulding.In document [5], it is specified that the nylon-11 liner is produced byrotomoulding. In document [6], it is mentioned that the liner isobtained from a thermoplastic which is extruded, blow moulded orrotomoulded. In documents [7] and [8], it is mentioned that thethermoplastic liner may be moulded by extrusion, blow moulding or byrotomoulding.

Injection moulding is rarely used due to technical limitations and pressand mould cost reasons. This is because the impermeable liners can havean internal volume of up to 150 litres, with thicknesses of severalcentimetres. Thermoforming is rarely used, although it is technicallypossible to use this technology to produce such impermeable liners.

The current technology of rotomoulding of molten thermoplastics has aparticular advantage. This is because it makes it possible:

-   -   to be able to manufacture large-size parts, ranging up to 150        litres, or even beyond;    -   to be able to insert one or more socket(s) (connecting pipe that        makes it possible to fill the liner with gas and to empty it)        and this being without bonding operations subsequent to        processing;    -   to provide thick and homogeneous impermeable liners.

The website [9] of the Association Francaise de Rotomoulage [FrenchRotomoulding Association] (AFR) describes a rotomoulding protocol viamelting of a thermoplastic.

In all these processes, the thermoplastic is melted in order to beshaped to the desired liner geometry, then it must be cooled beforebeing demoulded. Numerous defects of the liner result from this melting,especially the formation of networks, unmelted material ormicroporosities, and oxidation of the thermoplastic. These defectsimpair the final sealing performance of the liner, and therefore theperformance of the tank. Furthermore, in the case of rotomoulding, evenif bonding of the socket to the liner is not necessary, the sealingbetween the socket and the liner is not always satisfactory, due to thefluidity of the molten thermoplastic which is insufficient to closelymatch the forms of the socket. Moreover, this fluidity of the moltenmaterial cannot be increased by raising the temperature without causinga chemical impairment of said material. In addition, the processes usedtake a lot of time, which is further prolonged by the cooling time ofthe material after moulding the liner, especially due to the inertia ofthe mould.

Polyamide 6 (PA-6) is the thermoplastic which appears the mostadvantageous for the manufacture of impermeable liners, considering thecompromise between its barrier properties to gases, especially hydrogen,and its mechanical properties over a wide temperature range extendingfrom −40° C. to +100° C. Unfortunately, PA-6 is poorly suited torotomoulding because, as in other techniques for mouldingthermoplastics, it needs to be melted at a temperature above 223° C. inorder to give it the desired shape. The cooling time of the polymer isthen relatively long. In addition, this melting leads to the defectsidentified above which impair the final performance of the tank. Thedevelopment of thermoplastics, for example of PA-6, having grades moresuitable for rotomoulding, that is to say having a low water content ofthe powders, a lower viscosity, a suitable molecular weight, suitableantioxidants, etc., does not make it possible to solve all thesedefects. Furthermore, the development of the technology of rotomouldingmachines, for example rotomoulding under nitrogen, controlled cooling,reduction of the cycle time, does not make it possible to solve allthese defects either. This is because, for example, since melting of thePA-6 begins from around 220° C., this melting step is chemicallydegrading as the molten PA-6 must remain above its, melting point for 5to 15 minutes with process temperatures that exceed, sometimes by 40°C., the melting point.

Documents [10] to [15] show the current state of the art, thedevelopments in progress, and especially the thermoplastics and theirmode of implementation for developing type IV tanks, for a fuel cellapplication.

But none of these documents mention an element or envelope that isimpermeable to pressurized fluids, for example at a pressure of 10⁷ to10⁸ Pa, nor their manufacture. In addition, the compositions describedin these documents do not make it possible to obtain impermeableelements or an impermeable envelope having sufficient mechanicalproperties and barrier properties to pressurized gases, for example formanufacturing a type IV tank.

There are mainly two types of polymerization for obtaining polyamide 6:hydrolytic polymerization and anionic polymerization. Hydrolyticpolymerization cannot be used in reactive rotomoulding.

Documents [16], [17] and [28] to [32] describe caprolactam compositionsthat make it possible to manufacture objects moulded from polyamides orcopolyamides by anionic polymerization in a rotating mould: the polymeris formed by in situ polymerization, in the rotating mould. This istherefore referred to as reactive rotomoulding.

However, the caprolactam compositions described in these documents donot provide a truly satisfactory solution to the numerous aforementionedproblems. In addition, it is necessary to add nucleating agents(silica-type fillers, for example microtalc), plasticizers (for example,phthalates) or elastomeric units (for obtaining alloys), in order toimprove the mechanical properties and/or gas barrier properties. Theseadditions increase the time and the cost of manufacturing the envelope.Furthermore, the mixtures are complex to produce and their homogeneityis difficult to obtain.

There is therefore a real need for a composition that overcomes thesedefects, drawbacks and obstacles of the prior art, in particular for acomposition that makes it possible to control the manufacturing time,reduce the costs and improve the gas barrier properties, especially thebarrier properties to hydrogen, and the mechanical properties,especially low-temperature elastic deformation, of the envelopesmanufactured, and that makes it possible to obtain envelopes that arecompatible for use as an impermeable liner for a type IV tank.

This composition must enable, for example, the manufacture of a tankenvelope or liner for a low-temperature fuel cell (PEMFC) for which thestorage of the hydrogen, carried out at pressures ranging from 3.5×10⁷Pa to 7×10⁷ Pa, or even 10⁸ Pa, requires tanks that are light, safe andinexpensive, especially for onboard storage (transport).

DESCRIPTION OF THE INVENTION

The present invention specifically provides a composition that overcomesthe defects, drawbacks and obstacles of the prior art.

The composition of the present invention comprises in % by weightrelative to the total weight of the composition:

-   -   from 70 to 90% of an ε-caprolactam monomer of formula (I);    -   from 0.1 to 1% of an ε-caprolactam activator of formula (II), in        which R is chosen from the group comprising C_(n)H_(2n+2), n        being an integer chosen from 1 to 10, preferably from 1 to 6;        —OH; —OC_(n)H_(2n+2), n being an integer chosen from 1 to 10,        preferably from 1 to 6; and —NHR′ where R′ is either        C_(n)H_(2n+2), n being an integer chosen from 1 to 10,        preferably from 1 to 6, or an amine functional group;    -   from 2 to 6% of an ε-caprolactam catalyst of formula (III), in        which X is chosen from the group comprising MgBr, MgI, Li and        Na; and    -   from 10 to 20% of an ε-caprolactone additive of formula (IV);        in which the formulae (I), (II), (III) and (IV) are the        following:

By way of example, a composition comprising 80.6% of said ε-caprolactammonomer, 0.4% of said ε-caprolactam activator, 4% of said ε-caprolactamcatalyst and 15% of said ε-caprolactone additive has given excellentresults during numerous experimental trials carried out by theinventors.

The composition of the present invention makes it possible to obtainPA-6 polyamides by anionic polymerization of the .epsilon.-caprolactammonomer, added to which are an ε-caprolactam catalyst, for example ofbromomagnesium lactamate type, an ε-caprolactam activator, for exampleof acyl caprolactam type, and an ε-caprolactone additive, each beingused in a very precise range in terms of % by weight. This novelcomposition makes it possible to obtain a PA-6 polyamide, of which thebarrier properties to gases, especially to hydrogen, and the mechanicalproperties, especially low-temperature elastic deformation, are improvedrelative to the compositions of the prior art and, a fortiori, arecompatible with an improved use as an impermeable liner for type IVtanks.

These mechanical and/or gas barrier properties are improved withoutadding nucleating agents, plasticizers or elastomer units. This resultsin a not inconsiderable saving in time and cost in the manufacture ofimpermeable elements such as impermeable envelopes or liners. Moreover,considering the nature of the components of the composition of thepresent invention, the homogenization of the mixture is facilitated.

The ε-caprolactam monomer of the composition of the invention is acommon caprolactam also known as 2-oxohexamethyleneimine. Preferably, apure or practically pure product is used. It may be, for example, APCAPROLACTAME™ manufactured by DSM Fibre Intermediate, which is a 99.9%pure product.

The ε-caprolactam catalyst of the composition of the invention is suchas defined above, preferably a bromomagnesium lactamate. The activatoris such as defined above, preferably an acyl caprolactam. Thesecomponents have been chosen by the inventors of the present invention asthey allow a reaction that is gradual and controlled over a short time.This is because the viscosity of the mixture reaches its maximum (solidmaterial) at around 1 to 3 minutes (depending on the proportions) afterthe initiation of the reaction, which leaves time for the material to bedistributed over the walls of the rotomoulding mould. For informationpurposes, the ε-caprolactam activator, for example such as definedabove, may be in the liquid state at ambient temperature, whereas theε-caprolactam monomer and the ε-caprolactam catalyst (mixed withcaprolactam) may be in the form of flakes which melt at around 70° C.

The catalyst used may be, for example, a mixture ofbromo(hexahydro-2H-azepin-2-onato-N)magnesium (for example, from 10 to50% by weight) and ε-caprolactam (for example, 50 to 90% by weight),sold by Bruggemann Chemical under the trade name NYRIM C1 CATALYST. Theactivator used may be, for example, N-acteyl-hexanelactam, sold byBruggemann Chemical under the trade name AKTIVATOR 0. The ε-caprolactoneadditive used may be, for example, NYRIM ADDITIVE 6™ sold by DSM RimNylon VOF.

In fact, it is the ε-caprolactone additive which makes it possible toreplace the addition of nucleating agents, plasticizers or elastomerunits while retaining, or even improving, the mechanical and/or gasbarrier properties of the polymer obtained. This organic moleculedirectly participates in the polymerization reaction and is inserted inthe macromolecular chain in order to give it a greater “flexibility”.The composition of the present invention makes it possible, in practice,to form a random copolymer. This additive makes it possible to astutelyincrease the ductility of the PA-6 obtained without disturbing itsprocessing and its barrier properties.

The composition of the present invention may be used in the manufactureof an impermeable, tight, sealing element. The impermeable, tight,sealing element of the present invention has the ability to confine afluid, for example in a device, in a pipe or in a tank, that is to sayto prevent any exit of the fluid, whether under the effect or not of apressure applied to the fluid in contact with the impermeable element,during the entire operating life of the device, of the pipe or of thetank.

The term “fluid” is understood in the present description to mean aliquid or a gas or else a gas/liquid mixture. In each case, it may be apure liquid or a pure gas or a mixture of several liquids or severalgases. For example, in the case of a hydraulic or hydropneumaticaccumulator, two fluids may be confined, for example an inert gas and aliquid, for example nitrogen and mineral oil.

The term “element” is understood in the present description to mean anystructure that makes it possible to ensure sealing. It may be, forexample, a sealing joint; an impermeable, sealing, tight liner that mayor may not be self-supported; an impermeable, tight, sealing coating; animpermeable tight, sealing envelope; an internal or externalimpermeable, tight, sealing envelope of a tank; an impermeable, tight,sealing flask or canister; etc.

The present invention finds an advantageous application in theconfinement of pressurized fluids. In this case, the impermeable elementis an envelope that is impermeable, tight to pressurized fluids.

The use of the composition of the present invention is not limited toone particular process that makes it possible to obtain the impermeableelement. This is because the improved properties of the polymer obtaineddue to the composition of the present invention are inherent to saidcomposition. However, the composition of the present invention has acertain advantage for producing an impermeable envelope by rotomoulding.It is possible to use, for example, the composition of the presentinvention for manufacturing, by rotomoulding, a gas-impermeable envelopefor a composite type IV tank. One procedure that makes it possible toproduce such a tank is explained below.

Thus, the present invention also provides a process for manufacturing animpermeable envelope, said process comprising the following steps:

(a) preparation of a composition according to the invention and itsintroduction into a rotomould;(b) rotation of the rotomould and polymerization of the ε-caprolactammonomer of said composition to polycaprolactam, said composition beingheated to a polymerization temperature greater than or equal to themelting point of the ε-caprolactam and less than the melting point ofsaid polycaprolactam, so as to form said envelope by rotomouldingcoupled with a polymerization without melting of the polycaprolactamobtained;(c) crystallization of the polycaprolactam obtained; and (d) demouldingof the polycaprolactam liner obtained.

The polymer is formed at the same time as it takes up the shape of therotomould. Therefore, it is referred to as reactive rotomoulding since,strictly speaking, the rotomould is used both as a chemical reactor andas a mould that gives the shape of the liner. This reactive rotomouldingis particularly economical, fast and versatile. It leads to theproduction of materials that are more flexible than those of the priorart (rotomoulding by melting) and that have improved barrier properties,especially to gases.

The anionic polymerization reaction of the caprolactam monomer is anentirely conventional chemical reaction which makes it possible topolymerize a precursor monomer of a thermoplastic polymer to saidthermoplastic polymer. However, the development, of the “reaction path”of the present invention has required numerous research studies relatingto the formulation of the composition of the present invention, in orderto choose the activators, catalysts and additives that are most suitablein terms of the process (polymerization/moulding competition) and finaldesired properties. Numerous research studies have also been necessaryregarding the optimization of the rotomoulding procedures in order tointroduce the liquid components or their mixture and to control theanionic polymerization in the rotating mould.

Any method of preparing the composition of the present invention fromeach of its components is suitable, provided that the concentration ofeach of the components is such as defined in the present invention.Preferably, a preparation method will, of course, be chosen thatprevents the polymerization from being triggered before introduction ofthe mixture into the rotomould. The appended FIGS. 16A, 16B and 16Cpresent examples of the preparation of the composition of the presentinvention. Thus, for example, in step (a) it is possible to prepare twopremixes of said composition, one containing the monomer, the activatorand the additive, and the other the monomer and the catalyst, these twopremixes being mixed together just before, during or after theirintroduction into the rotomould to form said composition. The appendedFIGS. 16A and 16C illustrate examples of preparations of premixes. Thus,the two premixes can be prepared and stored (preferably, under a neutraland dry gas) separately several hours or even several days before themanufacture of the envelope and mixed together at the moment of theimplementation of the present invention. Also, for example it ispossible to simultaneously introduce the four components of thecomposition of the present invention into the mould (FIG. 16B).

Advantageously, according to the invention, in step (a), the compositionor the premixes is/are additionally preheated to a preheatingtemperature greater than or equal to the melting point of said monomerand less than said polymerization temperature, before or after theintroduction step (b) so as to melt the composition and homogenize it.The .epsilon.-caprolactam is liquid from 70° C. at atmospheric pressure.FIGS. 16A to 16C present this embodiment in various mixture and premixconfigurations. When one of the other components of the composition hasa melting point above the melting point of the monomer, the mixture orthe premixes is or are preferably preheated at least to said meltingpoint.

The premixes or mixture may be produced, for example, in a mixer forcaprolactam. In the case of the premixes, this mixer may be composed,for example, of two stainless steel chambers introduced into which are,on one side, a caprolactam+catalyst mixture and, on the other side, acaprolactam+activator+additive mixture. These two premixes may then behomogenized at the same temperature, typically from 100 to 150° C., forexample from 110 to 135° C., and stirred (preferably under an inert anddry gas) via an internal mechanical system. Each chamber isadvantageously equipped with a metering piston that makes it possible toinject a precise amount of each premix into the rotomould. The contactbetween the two premixes may take place outside of the mixer, forexample using a nozzle that then makes it possible to inject the mixtureobtained into the rotomould.

According to the invention, preferably, in step (a), at least theε-caprolactam monomer and the ε-caprolactam catalyst of said compositionare purged, just after weighing the bags and pots of these components,using an inert, preferably dry, gas due to their high sensitivity tomoisture. More preferably, the other components of the composition ofthe present invention are also purged. Preferably, the rotomould is alsopurged using an inert dry gas during the implementation of step (c). Theinert gas may be, for example, dry nitrogen. The inert gas is mostpreferably dry, so that the polymerization reaction is carried out in ananhydrous medium, in order to prevent the oxidation and uptake of waterof the ε-caprolactam components before the polymerization. In this case,the aforementioned mixer may advantageously be equipped with a systemfor sparging, for example with dry nitrogen.

The polymerization is initiated, at the polymerization temperature, assoon as the composition is formed, that is to say as soon as all thecomponents of the composition of the present invention are mixedtogether. It is therefore important not to waste time, at the risk ofseeing the mixture polymerize before its injection into the mould. Thevent through which the liquid mixture or the premixes are injected intothe rotomould may advantageously be modified in the shape of a funnel tofacilitate the introduction of the mixture or of the premixes.Advantageously, the injection of the two premixes may be carried outautomatically using a low-pressure injector or a high-pressure injector,for example having a scraper piston.

The amount of composition according to the invention introduced into therotomould determines, as a function of the size of the mould, thethickness of the wall of the envelope manufactured according to theprocess of the present invention. The choice of this thickness is mainlymade:

-   -   as a function of the desired barrier performance to the stored        gas, for example to hydrogen, of the polycaprolactam (for        hydrogen, drafts of standards ISO TC 197 and EIHP II which allow        a leakage of 1 cm³/litre of tank/hour);    -   as a function of the mechanical performance of the        thermoplastic, for example of sufficient resistance to the        installation of a mechanical reinforcement outside of the        envelope (liner), for example by winding of carbon fibres (the        envelope then serving as a mandrel) during the manufacture of a        tank; and    -   as a function of the mechanical performance of the        thermoplastic, for example a sufficient ductility in order not        to present fatigue during numerous filling operations of the        tank (greater than or equal to 1500 cycles between −40 and +85°        C.).

According to the invention, the envelope generally has a wall thicknessdefined so as to be able to withstand the leakage of the gas at thepressure at which it must be stored, known as the operating pressure,normally between 10⁷ and 10⁸ Pa (between 100 and 1000 bar). The presentinvention of course applies to pressures other than these, generallyfrom 10⁵ to 10⁸ Pa, the thickness of the envelope being chosen, inparticular, as a function of this operating pressure and the nature ofthe gas. In general, the thickness of the envelope is between 1 mm and60 mm, for example between 1 mm and 20 mm, for example between 2 and 10mm.

In the process of the invention, the polymerization is carried out in arotating mould or rotomould. For this, it is possible to use aconventional rotomoulding machine, for example such as those describedin the aforementioned documents relating to the rotomoulding of a moltenthermoplastic. Preferably, the mould of the rotomoulding machine issufficiently impermeable to the liquids, in particular to thecomposition of the invention minus its additive, since it then has aviscosity less than that of water. The rotomould may advantageously beequipped with one or more vents and an inlet for a neutral gas in orderto be purged using a dry inert gas for and/or during the implementationof the polymerization step (c) as explained above.

According to the invention, the rotomould is preferably rotated abouttwo axes (biaxial rotation), so that the polymerization takes place overthe entire internal surface of the mould provided to form the envelopeand conforming to this surface.

When rotomoulding molten material according to the prior art, therotational speeds of the primary axis and of the secondary axis arebetween 1 and 20 rpm (rpm: revolutions per minute), usually between 2and 10 rpm. In the process of the present invention, the rotationalspeeds (primary axis speed, secondary axis rotational speed and ratio ofthe speeds) are of the same order of magnitude, although the fluidity ofthe monomer is greater than that of the molten material. Thus, accordingto the invention, the rotational speed of the mould is preferably from 1to 30 rpm, more preferably from 2 to 25 rpm (revolutions per minute),about the primary and secondary axis. The ratio of the speeds (secondaryaxis speed/primary axis speed) is preferably equal to 7.5. Thesepreferred rotational speeds gave very good results with the compositionof the present invention.

According to one variant of the present invention, the rotomoulding maybe carried out in a rock and roll rotomoulding machine. The rotomould isin this case driven by a rotational movement about the longitudinal axisof the mould and by a rocking movement via which the two ends of themould are alternatively found at the top and at the bottom. Document[36] describes such a machine that can be used to carry out the processof the invention, and its use. Such rotomoulding may be useful, forexample, for manufacturing large-size and/or elongated tanks. Thecompositions and temperatures used are those of the present invention.

According to the invention, the polymerization is carried out at atemperature such that the envelope is formed without there being meltingof said polycaprolactam formed. This is because, if the melting point ofthe polymer formed is reached or exceeded during the polymerization ofthe monomer, this leads to the aforementioned defects of the liners ofthe prior art obtained by rotomoulding of molten material. According tothe invention, the step that consists in polymerizing the ε-caprolactammonomer to polycaprolactam or polyamide 6 (PA-6) in the rotating mouldis therefore carried out, most preferably, at a polymerizationtemperature of 150 to 200° C., preferably from 160 to 180° C.

For the same reasons as those explained above for heating of thepremixes and/or of the composition before introduction into the mould,preferably, according to the invention the rotomould may be heated to atemperature of 100 to 200° C., preferably 130 to 185° C., beforeintroduction of the composition of the invention. The mould may beheated, for example, using an oven into which the mould is introduced.When the mould has reached a temperature of 100 to 200° C., preferably130 to 185° C., the mould may be removed from the oven in order tointroduce the composition of the present invention thereto. It isoptionally possible to do without the oven by using, for example,infrared (IR) lamps, or a mould with integrated heating, for example viainfrared lamps or heat bands, or a double-walled mould with circulationof a heat transfer fluid.

In one advantageous embodiment, in particular for producing a thickimpermeable envelope according to the invention, it is possible torepeat the steps (a)+(b)+(c). This makes it possible to form animpermeable envelope having several layers of polycaprolactam(s) thatare identical or different, in thickness and/or in composition. Thus,starting from the same composition or starting from differentcompositions it is possible to carry out several successivepolymerizations and obtain a multilayer envelope according to theinvention. The compositions may be different in the concentration ofeach of the components and/or in the nature of the components of thecomposition, within the context of the definition of the composition ofthe present invention.

For example, in order to obtain envelope wall thicknesses greater than3-4 mm, advantageously several successive polymerization steps may becarried out until the desired thickness is reached. For example, it iseasy to make a polycaprolactam thickness of 6 mm in a single layer owingto the present invention, but to obtain a certain homogeneity inthickness, a thickness of 2 to 3 mm is preferable. Thus, for an envelopewall thickness of 6 mm or more, it is preferable to make, for example,several successive 3-mm layers of polycaprolactam.

Advantageously, since the polymerization reaction is exothermic, it isnot always necessary to put the rotomould back in the oven in order topolymerize each layer once the polymerization of the first layer hasstarted. This is because the polymerization of a first layer may beenough to maintain a sufficient temperature for the anionicpolymerization of the following layer.

The polymerization reaction time depends, in particular, on the natureof the activator and of the catalyst, on their proportions, on theprocessing temperature and on the size of the part to be manufactured.One of the many advantages linked to the composition of the presentinvention is that the polymerization reaction is very rapid, in generalfrom 2 to 10 minutes, often around 1 to 5 minutes. The preferredcompositions of the present invention make it possible to obtain, fromthe injection step to the crystallization step, the final polymer inaround 2 to 10 minutes, preferably in around 10 minutes. The completepolymerization reaction in fact takes place in four phases:

-   -   Mixing of the reactants: the viscosity is stable and low, the        components of the mixture have not yet reacted.    -   Polymerization: the monomers assemble to form polyamide        macromolecules which leads to an increase in the viscosity. The        macromolecules (polymers) are initiated starting from active        centres (activators) onto which the caprolactam monomers are        grafted. Thus, the molecular weight of the polyamide PA-6        obtained depends directly on the percentage of activator in the        reactive mixture. The reaction rate is itself a function, in        particular, of the proportion of catalyst. It is during this        phase that the homogenization of the part is critical.    -   Crystallization: the macromolecules reassemble to form        semicrystalline structures, this is the germination, followed by        the growth, of spherulites. The appearance of ordered zones in        the viscous medium renders the latter cloudy (light scattering).    -   Shrinkage, identified by a detachment between the material and        the mould. The shrinkage marks the end of the crystallization.

The chemical equation for the polymerization is shown schematicallybelow. In the formulae of this scheme, the bonds between the C and Natoms are covalent bonds. They are represented by dotted lines toindicate that they preferably open during the polymerization. “p” is thedegree of polymerization of the polyamide PA-6. This degree ofpolymerization may be 1≦p≦100 000.

When the polymerization is finished, in particular when the length ofthe chains is sufficient and the crystallization is completed(organization of the polymer chains), it is optionally possible to coolthe mould for a few minutes, in particular to facilitate the handling ofthe envelope manufactured, in order to prevent any risk of burns. Theenvelope is then demoulded. This results in an obvious time savingcompared to the processes of the prior art, especially considering theinertia of the mould, where the temperature of molten rotomoulding ofthe prior art was much higher than that used in the process of thepresent invention, and where it was necessary to wait for the materialto change from the melt state to the solid state.

The process of the invention allows the manufacture of a polycaprolactamimpermeable envelope that is capable of being incorporated into themanufacture of any composite tank intended for the storage of fluids(liquids or gases or liquid+gas mixtures), in particular pressurizedgases. The impermeable envelopes manufactured by the process of theinvention have higher performances in turns of mechanical and gasbarrier properties than those of the prior art, especially since thereis no longer a risk of chain scission, of oxidation, of crosslinking, ofpolycondensation, of final porosity, of residual stresses orinhomogeneity, etc., that are inherent to the melting and solidificationphenomena of thermoplastic polymers.

In addition, as shown by the experimental results explained in theexamples below, the interior surface condition of these envelopes ismuch better than that of liners obtained by a molten material process ofthe prior art or with polymerization compositions of the prior art.These improved properties are obviously reflected in all the propertiesof the tanks that are manufactured from these envelopes.

According to the invention, the envelope obtained may additionally besubjected to one or more post-treatment(s) intended to coat its inner orouter surface with one or more thin film(s) in order to further improvethe sealing properties of the liner to the gas which will be storedtherein (barrier properties) and/or to give it particular chemicalproperties, for example resistance to chemical attack, a food gradequality or a better ageing resistance. This post-treatment may consistof a deposition treatment of SiO_(x) type, where 0≦x≦2, or else ofSi_(y)N_(z)C_(t) type, where 1≦y≦3, 0.2≦z≦4 and 0≦t≦3, viaplasma-enhanced chemical vapour deposition (PECVD), of aluminium via aphysical vapour deposition (PVD), deposition of epoxy type by chemicalcrosslinking, or fluorination with CF₄, for example. Documents [21] and[22] describe this type of post-treatment that is well known to a personskilled in the art in the manufacture of type IV tank liners, and thatcan be used on the liner obtained by the process of the presentinvention.

According to the invention, it is possible to attach at least one tanksocket to the inside of the rotomould before carrying out step (c) sothat the tank socket is incorporated into the impermeable envelopeduring the polymerization. This makes it possible, in particular, tomanufacture a tank with or starting from said envelope (for example, animpermeable liner). When the envelope manufactured is small (forexample, for a small tank) a single socket may be sufficient. For alarge-size envelope (for example, for a large-size tank), it ispreferred to install two sockets, in particular to allow rapid fillingand emptying of the tank. The socket (or the sockets) may be installedat one end (at both ends) of the envelope, in particular when it has anelongated shape, but also on the length of the envelope, somewherebetween the ends.

According to the invention, said at least one metal socket provides theinterior/exterior connection of the tank for its filling and for the useof the stored gas. The socket may be a socket conventionally used forthis type of tank, for example an aluminium or steel socket. One or moresocket(s) may be positioned in the mould to obtain one or more socketson the manufactured envelope. The socket or sockets may be subjected toa treatment intended to further improve the sealing of thesocket/envelope join. It may be, for example, a chemical treatmentconsisting in depositing, on the socket, a specific or silane epoxyresin in order to improve the chemical attachment of the PA-6 obtainedvia the polymerization in the process of the invention. This treatmentmay be, for example, a treatment such as that described in document [4].According to the invention, prior to this deposition of resin, thesocket may be sandblasted or undergo an acid treatment in order tofurther improve the adhesion of the resin and therefore of the envelopeon the socket.

The inclusion of one or more socket(s) on the envelope may be carriedout according to the conventional processes well known to a personskilled in the art, for example according to the processes described indocuments [4] and [23], or in one of the aforementioned documents whereat least one socket is provided. However, in the present invention thethermoplastic polymer is not melted in order to be joined to the socket;it is formed by polymerization of the monomer simultaneously in themould and on the socket or sockets positioned in the mould beforerotomoulding according to the process of the present invention. Thesocket or sockets may be positioned, for example, in the mannerdescribed in document [23].

The envelope obtained according to the process of the invention,equipped with the socket or sockets, is then demoulded. Due to theprocess of the present invention, the risk of leakage at the sockets isgreatly reduced, as during the rotomoulding the viscosity of the monomerat the start of the polymerization is very low and it very readilyspreads into the interstices and/or attachment points of the socket.

The present invention also relates to a composite tank storing a fluid,said tank comprising an impermeable envelope capable of being obtainedby carrying out the process of the invention.

The fluid may be such as defined above, for example a pressurized gas.The present invention finds an application for storing any pressurizedor unpressurized fluid, for example gaseous hydrogen, helium, naturalgas, compressed air, nitrogen, argon, hytane, etc.

For example, said tank may comprise, in this order, from the inside ofthe tank to the outside of the latter, at least: [0082] said impermeableenvelope (2); [0083] at least one metal socket (4); and [0084] an outermechanical reinforcement (6) for the envelope.

The envelope may be such as defined above. In this type of tank, it isusually known as a “liner”.

The socket or sockets may be such as defined above. When there areseveral sockets, they may be identical or different.

According to the invention, the outer mechanical reinforcement of theenvelope provides the mechanical strength of the tank. It may be any oneof the reinforcements known to a person skilled in the art commonlypositioned around the envelopes of tanks, for example of type III or IV.It may be, for example, a filament winding. This filament winding may becomposed, for example, of carbon fibres and thermosetting resin. Forexample, the carbon fibre, previously impregnated with uncrossed epoxyresin, may be wound around the envelope held by the socket or sockets,for example according to one of the processes described in documents[4], [5], [24] or [25]. The envelope, which in the particular example ofa type IV tank, is a self-supported structure, in fact serves as amandrel for this filament winding. A type IV tank may thus be obtained.

The envelope manufactured according to the invention therefore makes itpossible to obtain a composite type IV tank, the mechanical and barrierperformances of which are much greater than those of one and the sametank whose liner (made from the same thermoplastic) is manufactured byextrusion-blow moulding, thermoforming, injection moulding orrotomoulding of the molten thermoplastic or of a polymerizationcomposition of the prior art.

The present invention is particularly suitable for the manufacture of atank that supplies fuel cells, in particular low-temperature fuel cells,for which the mechanical requirements are very strict, andhigh-temperature fuel cells, for which the sealing requirements are alsovery strict. The fact of using compressed H₂ tanks for PEMFCs,especially for transport applications (for example, car, bus, etc.)necessitates having sufficient autonomy, that is to say loading as muchH.sub.2 as possible, which is carried out by increasing the operatingpressure of the tank up to 7×10⁷ Pa (700 bar) and even higher.Furthermore, for transport applications, the tanks must preferably belight, which implies the use of composite type III or IV tanks.

Due to the composition of the present invention and to the process ofthe present invention, the envelope may have a thickness such that itwithstands a tank operating pressure between 10⁷ and 10⁸ Pa (between 100and 1000 bar). The composition of the present invention may thereforeadvantageously be used for manufacturing a type III or IV tank, forexample such as those mentioned previously.

The composition of the present invention and the process formanufacturing the envelope also make it possible to manufacture animpermeable envelope that can be used for manufacturing hydraulic orhydropneumatic accumulators. This is because such an envelopeadvantageously withstands variable pressures which may range fromatmospheric pressure (10⁵ Pa) to 10⁸ Pa.

Hydraulic or hydropneumatic accumulators and their uses are known to aperson skilled in the art. One of the main functions of theseaccumulators is to store energy transmitted by a liquid in the form ofvolume and pressure and to restore it automatically or on demand. Theirfunction may be one or more of the following functions: to accumulateenergy and distribute this energy at a desired power, so as to make itpossible to reduce an installed power; to absorb pressure pulsationsproduced by a pump; to compensate for leaks by forming a pressurereserve; to absorb variations in volume of a liquid, especially causedby temperature differences in a liquid circuit and to constantly keepthe circuit pressurized; to fully transmit pressures from one fluid toanother without risk of mixing.

A description of such accumulators and of their uses is presented, forexample, in Techniques de l'Ingénieur, A767, “Appareillage de controledes fluides dans les tuyauteries” [Equipment for the control of fluidsin duct work], by Jean Sutter, or else on the Internet sites referenced[34] and [35] in the appended list of references. They generallycomprise an outer body that provides the mechanical strength and sealingof the accumulator, a flexible internal pocket which may contain, in animpermeable manner, a generally neutral gas (for example, nitrogen). Thepocket is not bonded to said body and forms a space that allows a fluidto be introduced around this pocket. The accumulator also comprises afirst connector that makes it possible to fill or empty said internalpocket with said gas, and a second connector used to introduce a fluidbetween said pocket and the body of the accumulator during its use. Thestorage of the energy stored in the accumulator is carried out bycompressing the gas contained in the flexible inner pocket viaintroduction of the fluid through the second connector. The pocket makesit possible to isolate the gas that it contains from the fluid thatsurrounds it, in order to avoid any dissolution or entrainment of thegas in the fluid, that is to say a reduction in the amount of gaspresent in the inner pocket of the accumulator, a drop in the inflationpressure, and an increase in the compressibility of the fluid, thatleads to a disruption in the operation of the accumulator and of thedevice which uses this accumulator.

In the accumulators of the prior art, the body is formed from a metal(carbon steel or stainless steel) or plastic (polyvinyl chloride,polypropylene or polyvinyldiene fluoride) shell with metallicreinforcement depending on the use (field, usage pressure, etc.). Theinner pocket is made from one and the same part. This pocket isgenerally formed from nitrile rubber (NBR); it may also be formed from amaterial chosen from isobutene-isoprene rubber (IIR), epichlorohydrinrubber, ethylene propylene rubber, natural rubber, the elastomer forfood usage, the nitrile for hydrocarbons, etc. The fluid surrounding theenvelope is generally chosen from water, mineral oil, or gas (forexample nitrogen or another neutral gas).

The accumulators of the present invention differ from those of the priorart, for example from those described in the aforementioned document andInternet sites, in that, in the present invention, the body of theseaccumulators from the prior art is replaced by the impermeable envelopeof the present invention, equipped or not with an outer mechanicalreinforcement such as defined above for type IV tanks (for example, afilament winding). The mechanical reinforcement will be used, forexample, when the usage pressures of the accumulator require it.

In other words, the accumulator of the present invention may be, forexample, in the form of a type IV tank according to the presentinvention, this tank being additionally equipped with a flexible innerpocket and the aforementioned connector. The connector, the flexiblepocket, the gas in the pocket and the fluid around the pocket may bechosen from those known to a person skilled in the art, for example fromthose cited in the aforementioned document and Internet sites. Theseelements may be, for example, such as those cited above.

More generally, the composition of the present invention and theimplementation process via rotomoulding may be used for variousapplications such as: [0097] impermeable liner for a type IV tank;[0098] inner coating for an insufficiently impermeable liner of a typeIV tank (provision of the gas-barrier property); [0099] inner coatingfor a metal liner, for example made of aluminium or of steel, for a typeIII tank (provision of the gas barrier property to limit the effects ofembrittlement or of the water-barrier property to limit the effects ofcorrosion); and [0100] inner coating for a type I or type II tank; etc.

Thus, the specific formulations of the composition of the presentinvention processed by rotational moulding may be used each time that abarrier property (liquid or gas or liquid+gas mixture) is desired,optionally with a good mechanical flexibility (elastic deformationrequired without fatigue) and optionally with a thermomechanicalstrength over a wide temperature range, typically from −60° C. to +110°C., without impairment of the preceding properties.

The reactive rotomoulding used in the present invention makes itpossible to produce a finished product rapidly (in a few minutes) and ina single step, compared to four steps by the molten route of the priorart. The rotomoulding step is facilitated due to lower temperatures(little oxidation, chain scission, risk of polycondensation, etc.) andan environment (preferably, an inert and dry atmosphere) that is lesscritical than in the processes of the prior art. The industrializationis therefore easier. Furthermore, the final PA-6 synthesized in situ hasimproved properties as can be seen throughout the examples below.Finally, it is easier to very readily modify the final properties of thepolymer through the choice and the amount of activator, catalyst andadditives used.

Other advantages may yet appear to a person skilled in the art onreading the examples below, illustrated by the appended figures, thatare given by way of illustration.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 represents a typical curve of temperature (T in ° C.) as afunction of time (t in minutes) for reactive rotomoulding using thecomposition of the present invention (from the injection onwards, themould remains outside of the oven).

FIG. 2 represents a curve of temperature (T in C) as a function of time(t in minutes) for reactive rotomoulding with rapid polymerization usinga composition according to the present invention in which, relative tothe composition used to obtain the curve from FIG. 1, a larger amount ofcatalyst and of activator is added (from the injection onwards, themould remains outside of the oven).

FIG. 3 represents a curve of temperature (T in ° C.) as a function oftime (t in minutes) for reactive rotomoulding with belatedcrystallization (low catalyst content, presence of additives disruptingthe crystallization) (from the injection onwards, the mould remainsoutside of the oven).

FIG. 4 represents a curve of temperature (T in ° C.) as a function oftime (t in minutes) for three-layer reactive rotomoulding obtained byrepeating steps (a), (b) and (c) of the process of the invention severaltimes (from the first injection onwards, the mould remains outside ofthe oven).

FIG. 5 represents a section of a 3 litre liner: on the left aheterogeneous part obtained with a caprolactam composition and processparameters of the prior art; on the right a homogeneous part obtaineddue to the composition of the present invention.

FIG. 6 is a photograph of a cross-sectional view of a three-layerenvelope (layers 1, 2 and 3) obtained from the composition of thepresent invention: mould side on the left, internal air side on theright.

FIG. 7 is a photograph of the spherulites from layers 1, 2 and 3 of thethree-layer envelope from FIG. 6.

FIG. 8 is a photograph of the interphase between layers 2 (on the left,area with large spherulites) and 3 (on the right) of the three-layerenvelope from FIG. 6.

FIG. 9 represents a graph (energy W/g as a function of the temperature Tin ° C.) that collates the results of differential scanning calorimetry(DSC) analyses on rotomoulded PA-6 obtained by a molten route accordingto the prior art (broken line); on rotomoulded PA-6 obtained by areactive route with a composition of the prior art without an additive(fine line) and on rotomoulded PA-6 obtained by a reactive route with acomposition of the present invention (with additive) (heavy line).

FIG. 10 represents a graph (stress (σ) in MPa as a function of strain(ε) in %—uniaxial tensile curve at 20° C.) showing the influence of theactivator content in the composition of the present invention on theelongation at break.

FIG. 11 represents a graph (stress (σ) in MPa as a function of strain(ε) in %—uniaxial tensile curve at 20° C.) showing the influence of theadditive concentration in the composition of the present invention(tests at 0 (dotted line), 10 (heavy line) and 15% (fine line) by weightof the composition) on the mechanical characteristics of a rotomouldedenvelope (stress (σ) in MPa, as a function of the strain (ε) in %). Forcomparison, a curve resulting from equivalent tests carried out on apolyethylene envelope is also represented.

FIG. 12 is a graph showing the change in the permeability coefficient(Pe) measured for various PA-6 samples manufactured by reactiverotomoulding from the composition of the present invention (hydrogenpermeability coefficient at 27° C.) (Pe H₂ at 27° C.) in mol/(mPas)).

FIG. 13 is a Datapaq™ curve of the temperature (T in ° C.) cycle as afunction of time (t in minutes) for a 22 L liner made of PA-6 obtainedby reactive rotomoulding (injection of layer 1 (C1) at t=0) startingfrom a composition according to the invention.

FIG. 14 schematically represents an example of the structure of a typeIV tank (1) manufactured from an envelope (2) obtained from acomposition according to the present invention. This figure representsthe functionalities of the various components which form this tank.

FIG. 15 shows 7 photographs taken during the manufacture of 22 litreimpermeable envelopes starting from compositions and the processaccording to the invention.

FIGS. 16A, 16B and 16C schematically present various mixing methods thatmake it possible to obtain the composition of the present invention thatcan be used for implementing the process of the present invention.

FIG. 17 schematically represents an example of the structure of ahydraulic accumulator (10) manufactured from an envelope (2) obtainedfrom a composition according to the present invention. This figurerepresents the functionalities of the various components which form thisaccumulator.

EXAMPLES Example 1 Example of Components for Preparing a CompositionAccording to the Present Invention

The components used in the examples below for preparing compositionsaccording to the present invention were the following:

-   -   ] ε-caprolactam monomer (I): AP CAPROLACTAME™, supplier: DSM        Fibre Intermediate B.V., melting point: 69° C.;    -   ε-caprolactam activator (II): AKTIVATOR 0™, supplier: Bruggemann        Chemical, form: liquid, melting point: −13° C.;    -   ε-caprolactam catalyst (III): NYRIM 1 CATALYST™ as        catalyst (III) (bromure        de(hexahydro-2H-azepin-2-onato-N)magnesium, supplier: Bruggemann        Chemical, form: flakes at ambient temperature, melting point at        70° C.; and    -   ε-caprolactone additive: NYRIM ADDITIVE 6® as additive (IV) sold        by DSM RIM NYLON VOF, form: liquid at ambient temperature,        boiling point around 260° C. at 760 mm Hg, melting point below        0° C.

The chemical formulae of these components (I), (II), (III) and (IV) werethe following:

In the examples below, they are respectively referred to as “monomer”,“activator”, “catalyst” and “additive”.

The appended FIGS. 16 a to 16 c show three examples for preparing andpreheating premixes starting from these components, before introductioninto the rotomould to carry out the polymerization.

Example 2 Example of Devices for Implementing the Process of theInvention

To produce premixes as indicated in the appended FIG. 16 a or 16 c, aBronk™ mixer, model NCU 75/6 was used. This mixture was composed of twochambers (compartments) made of stainless steel introduced into whichwill be, on one side, a caprolactam+catalyst mixture and, on the otherside, a caprolactam+activator+additive mixture. An internal mechanicalsystem for each chamber made it possible to mix its two premixes and topreheat them.

Each chamber of the mixer was equipped with a system that made itpossible to inject dry nitrogen inside the chambers in order to degasthe components used and to keep them under a dry and inert atmosphere.

Each chamber of the mixer was also equipped with a metering piston thatmade it possible to inject a defined quantity of each premix into therotomould.

Mixing or contact between the two premixes was carried out outside themixer using a double-channel nozzle that made it possible to inject themixture obtained into the rotomould.

The rotomould used in these examples was of the Shuttle type, from thebrand STP Equipment, and referenced LAB40. In order to heat therotomould, an electric oven was used. Vents were provided for spargingwith dry nitrogen during the implementation of the rotomouldingprocesses.

Before injection of the components, premixes or of the mixture into therotomould, an aluminium socket was attached to the mould, in the mannerdescribed in document [23], after optionally having been subjected to atreatment, for example such as that described in document [4].

The socket was equipped with a funnel that facilitated the introductionof the components or of the premixes into the rotomould.

Example 3 Production of an Envelope and Tests on the Composition of theInvention

The composition used in this example was the following in % by weight:80.6% of ε-caprolactam monomer, 0.4% of activator, 4% of catalyst and15% of additive. In this example, the total amount of composition usedto form the envelope was 397 g. The volume of the envelope formed was 3L.

The ε-caprolactam monomer and the ε-caprolactam catalyst were purgedjust after weighing of these components, using dry nitrogen.

The components were preheated, mixed together, then rapidly introducedinto the rotating rotomould as shown in FIG. 16 b. The rotomould washeated in an oven up to a temperature of 160 to 190° C.

The mould was rotated at a rotational speed of the primary axis of 9 rpmand of the secondary axis of 6 rpm. The ratio of the rotational speeds(primary/secondary ratio) was equal to 1.5.

The appended FIGS. 1 to 3 collate various temperature measurementscarried out in this example. On these figures, the change in thetemperature of the mould (Tm) and the change in the temperature of theair inside the mould (Ti) are shown. The hashed zones indicate theperiod during which polymerization took place and the dotted zonesindicate the period during which crystallization took place. The momentof the injection of the composition into the rotomould is shown by thereference “I” and “RR” shows the beginning of the drop in temperaturesubsequent to the cooling of the mould (rapid cooling). “Fc” indicatesthe end of the crystallization. For the three figures, starting from theinjection, the mould remains rotating outside of the oven.

Represented in FIG. 1 are typical temperature curves for reactiverotomoulding starting from the aforementioned composition. From theinjection onwards, the mould remained outside of the oven. The exothermsrepresented in this figure (increase in the temperature) are thereforemarkers of the polymerization and crystallization steps.

Represented in FIG. 2 are temperature curves for rotomoulding using theaforementioned composition in which, relative to the composition used toobtain the curve from FIG. 1, a larger amount of catalyst and ofactivator is added. This larger amount corresponds to 1.5 times for eachof these components. In this case, the polymerization reaction isfaster, the two exothermic peaks may be superposed. Nevertheless, thepolymerization is separate from the crystallization in the form of ashoulder (E) at the crystallization peak.

Represented in FIG. 3 are temperature curves for reactive rotomouldingwith belated crystallization: low catalyst content, presence ofadditives disturbing the crystallization. These additives were 10% byweight of a silica type additive. It is remarked that here, thecrystallization started in the cooling phase only (after “RR”).

The temperature curves (curves obtained using a Datapaq™ system) for areactive rotomoulding cycle were therefore completely different fromthose of a conventional rotomoulding cycle (melting of the thermoplasticpolymer).

These curves allow precise monitoring of the in situ anionicpolymerization and are an invaluable tool for implementing the presentinvention. From these curves, the process used in this example may bedescribed in the following manner:

-   -   1—Heating of the empty mould in a neutral and dry atmosphere:        the internal air temperature follows, some fifteen or so degrees        below, the change in the temperature of the mould. The heating        is faster than for the molten route as the mould is empty.    -   2—Removing the mould from the oven and stopping the rotation.    -   3—Injecting the mixture into the mould: the internal air        temperature drops due to the lower temperature of the mixture.    -   4—Rotating the mould: the internal air temperature rises until        it stabilizes around 15° C. below the temperature of the mould        at the moment of injection. The stabilization time corresponds        to the polymerization activation time.    -   5—Polymerization characterized by an increase in the internal        air temperature (exothermic reaction) that is more or less rapid        depending on the catalyst content.    -   6—Crystallization characterized by a second exothermic peak.    -   7—Rapid cooling that is optional depending on the demoulding        temperature, conventionally from 80 to 40° C.    -   8—Stopping the rotation, opening the mould and demoulding the        envelope formed.

The envelope thus formed is neither oxidized nor crosslinked; thepolymer has not undergone chain scission (cut) and has neither unmeltedmaterial nor residual porosity. Tests showing the physical properties ofthese parts are explained below.

Example 4 Production of a Multilayer Envelope According to the Invention

In this example the inventors produced “thick” parts by superposition ofseveral polymer layers starting from the composition and from theprocess of the present invention. This could not be carried out with thecompositions of the prior art.

In this example, an impermeable liner having a thickness of 6 mm (3successive layers of 2 mm), and internal volume of 22 litres (waterequivalent) and equipped with two metal sockets was produced.

Two premixes were produced as represented in FIG. 16C using theaforementioned mixer first placed under dry nitrogen and at atemperature of 135° C. Introduced into one of the two compartments were:1441.5 g of monomer and 143.1 g of catalyst. Introduced into the othercompartment were: 1441.5 g of monomer, 536.6 g of additive and 14.31 gof activator.

After homogenization and preheating of each of the two premixes, theinjection of 1192.3 g of the mixture (a proportion of each of thepremixes: ⅓ from each compartment) into the mould, that had previouslybeen heated to 165° C., was carried out through the vent of therotomould, using the funnel, to form the first of the three layers.

The mould was rotated with a rotational speed of the primary axis of 6.2rpm and of the secondary axis of 23.3 rpm. The ratio of the rotationalspeeds (primary/secondary ratio) was equal to 0.27.

The mould was kept at temperature in the oven for the three layers inorder to obtain a very good repetition of the temperature cycle of eachlayer, as can be seen in the graph from appended FIG. 13 showing thechange in the temperature (mould, socket and internal air) as a functionof the time (t in minutes). This figure shows a Datapaq™ curve of thetemperature cycle during the manufacture of this 22 L PA-6 linerobtained by reactive rotomoulding (injection of layer 1 (C1) at t=0)according to this example. The temperature peaks ranging from 80 to 156°C. correspond to the temperature measurements of the air outside therotomould during rotomoulding using two temperature sensors (curves 1and 2). The other curves show the change in the temperature of the mould(curve 3), of the socket (curve 4) and of the internal air (curve 5).

Thus, from a composition according to the invention and from a processsuch as described in Example 2, the steps of preparation of thecomposition, and of polymerization and crystallization in the rotomouldwere carried out three times in a row.

Each layer was injected into the mould as soon as the preceding layerhad a sufficient viscosity so as not to flow when the rotation of themould was stopped and before its complete polymerization in order toensure an optimal adhesion with the following layer. This step thereforetook place when the polymerization was sufficiently advanced, that is tosay between 2 and 5 minutes after the injection of the preceding layerinto the rotomould. Due to the exothermic nature of the polymerizationand the crystallization, and therefore due to a cooling that was overallvery slow, it was therefore possible to produce numerous layers. Thus,there was almost no limit to the final thickness.

In this example, the total amount of composition used was 3577 g.

After cooling, the envelope formed was removed from the mould.

The parts obtained are shown in photographs in FIG. 15. In the top left:Bronk™ injection system and neutral gas (nitrogen); in the top middle:envelope obtained with socket during extraction from the rotomould; inthe top right and middle left: envelope during the complete cooling.Central photo and middle right photo: inside of the envelope on thesocket side and after cutting of the dome covering the socket. At thebottom, two 22 L envelopes (liners) according to the invention with two2×10⁷ Pa (200 bar) aluminium sockets. The sockets/envelope connection isvery tight, contrary to that of an envelope obtained with a moltenrotomoulding procedure according to the techniques of the prior art.

The thermoplastic polymer which was obtained was polyamide 6 (orpolycaprolactam) that has the properties indicated in the furthestright-hand column in the comparative table below:

Type of rotomoulding Molten route Reactive route grade Flexible FlexibleStandard modified Standard modified PA-6 PA-6 PA-6 PA-6 PolymerizationHydrolytic Anionic Molar mass kg/mol 15-50 50-300 Density g/cm³ 1.131.15 1.14 Melting point ° C. 227 227 225 200 Elastic modulus Mpa 1400400 1900 700 Tensile strength Mpa 65 40 75 45 Elongation at break % 30nd 70 >300

The molten route PA-6 used was TECHNYL C217™ from Rhodia EngineeringPlastics (France). The flexible modified PA-6 was obtained by additionof elastomer units. The standard (reactive route) PA-6 corresponded to acomposition that did not contain an .epsilon.-caprolactone typeadditive.

The standard anionic PA-6 had, in the main, a higher performance thanthe standard hydrolytic PA-6 of the prior art via molten route (moltenroute column) (elastic modulus, tensile strength and elongation at breakwere better). Furthermore, the compositions of the present invention(reactive route-flexible modified PA-6) made it possible to obtainmechanical properties that were closer to polyethylene (PE) andtherefore more in adequacy with the desired properties for the type IVtank liner application (desired ductility).

Example 5 Other Multilayer Tests According to the Invention

With a composition and a procedure as in Example 4, a 3 L three-layerstructure was prepared. It was not necessary to put the rotomould backinto the oven for each layer, since the polymerization reaction wasexothermic it generated a sufficient temperature for the anionicpolymerization of the following layer taking into account the smallamount of material to be polymerized.

The appended FIG. 4 represents a temperature curve (T expressed in ° C.)for the three-layer reactive rotomoulding as a function of time (texpressed in minutes) obtained in this example, by repeating the steps(a), (b) and (c) of the process of the invention several times (from thefirst injection onwards, the mould remained outside of the oven). Thecurve with the heavy line represents the change in the internal airtemperature in the mould and the curve with the fine line represents thechange in the mould temperature. In this figure the references I1, I2and I3 respectively represent the first, second and third injections,and the hatched zones respectively represent, from the left to theright, the polymerization times of the first, second and third layers.The dotted zone itself represents the crystallization time. RR indicatesthe temperature drop subsequent to the rapid cooling of the mould.

In this example, each polymer layer had a thickness of 2 mm and thetotal thickness was 6 mm.

Example 6 Examination of the Envelopes Obtained According to theInvention

The first reactive rotomoulding tests were carried out with formulationsdeveloped for injection moulding (RIM: Resin Injection Moulding). Theseformulations had the advantage of having very short reaction times (lessthan 3 minutes to crystallize). They were however incompatible with therotomoulding process. Thus the polymer did not have time to cover thewhole of the mould before forming and, a fortiori, setting (it should berecalled that this was well below the melting point of the polymer).This resulted in a very high heterogeneity of the thickness, with wallportions that were very thin (a few tenths of a millimetre) and ridgeshaving a thickness of a few centimetres.

Thus, the first part of the development carried out by the presentinventors was to find parameters (composition and process) that made itpossible to slow down the polymerization until a sufficient homogeneitycould be obtained. Many difficulties were encountered due to the factthat all the parameters were interdependent: catalyst content, activatorcontent, mould temperature, rotational speed, time, change in theviscosity, etc.

The appended FIG. 5 moreover makes it possible to visually compare(photographs) 3 1 envelopes seen in cross section: [0175] on the left, aheterogeneous part obtained with a caprolactam composition of the priorart and a rotomoulding process; and [0176] on the right, a homogeneouspart obtained thanks to the composition of the present invention, withthe same rotomoulding process.

Quite obviously, the composition of the present invention makes itpossible to achieve the desired objectives.

Analyses of cross sections were carried out on multilayer envelopesobtained as in Example 4. The bonding problems that can be encounteredin “molten route” multilayers of the prior art were nonexistent for theenvelopes obtained in accordance with the present invention since thepolymerization was continuous from one layer to another. However, it ispossible to visually distinguish the layers from one another due totheir difference in transparency.

The photograph from FIG. 6 represents a cross section of the three-layerstructure obtained (3×2 mm): mould side on the left, layer 1 (C1), layer2 in the middle (C2) and layer 3 on the internal air side of therotomould on the right (C3).

The difference in transparency of the three layers can be explained bythe size of the spherulites which compose them. This is because thelatter are large for layer 1 (C1) (around 20 .mu.m), medium for layer 2(C2) (around 10 μm), and small for layer 3 (C3) (around 5 μm) as can beseen in the photograph from FIG. 7. These differences can be attributedto the decrease in heat transfer between the mould and the polymer asthe layers are formed.

Furthermore, from the photograph of appended FIG. 8, a transition zonein terms of the size of spherulites between layer 2 (on the left—zonewith medium spherulites) and layer 3 (on the right-zone with smallspherulites) can be observed. This zone belongs to layer 2 but has beenmodified by the injection of layer 3. This continuous interphaseparticipates in the mechanical cohesion of the assembly since themacromolecular chains of each layer are tightly bound andinterpenetrated with the adjacent layer or layers.

Example 7 Properties of Envelopes Obtained from a Composition of thePresent Invention

In the experiments from this example:

-   -   The composition of the invention was that from Example 3.    -   The PA-6 used for the molten route was TECHNYL C217™ from Rhodia        Engineering Plastics (France).    -   The additive-free composition was the same as that from Example        3, but without an ε-caprolactone additive.

A. Microstructure and Crystallinity

Besides the melting points and the degrees of crystallinity,differential scanning calorimetry (DSC) tests make it possible todemonstrate various particularities of the polymer formed: for example,various crystalline phases, the presence of unpolymerized caprolactam,water uptake, postpolymerization and, generally, the polymerizationstate of the PA-6.

The appended FIG. 9 represents a graph that collates the results of DSCanalyses on rotomoulded PA-6 obtained by the molten route of the priorart (VF); on rotomoulded PA-6 obtained by the reactive route with anadditive-free composition from the prior art (Vra), and on rotomouldedPA-6 obtained by the reactive route with a composition of the presentinvention (VRI) (heavy line).

It is firstly noted that the temperature of the melting peak of the PA-6obtained by the reactive route (additive-free) was (slightly) lower thanthat of the melting peak of the PA-6 obtained by the molten route. Thisshows that this material has slightly thinner lamellae, which isfavourable for the barrier property to fluids, including gas, and forthe mechanical property (more ductile material).

The additive used according to the present invention acted on themonomer during the polymerization. It was integrated into themacromolecule (polymer) by forming a sort of block copolymer, providingan unexpected mechanical flexibility effect. This modification of themolecule modified the crystallization and therefore modified the finalproperties of the envelope obtained. On a practical level, the additive6 was used with the activator, which it partly consumed. Very liquid atambient temperature, it had the advantage of mixing very readily withthe caprolactam and of being able to be used at contents greater than10% by weight. The additive was tested at contents of 5%, 10% and 15% byweight, without significant modification of the heating cycle and of thepolymerization (the ratio between the other components not having beenmodified). On the other hand, the appearance and the mechanicalproperties were greatly and favourably modified, which was the desiredeffect.

The DSC measurements show, in this figure, that the parts obtained withthe composition of the present invention (heavy line) had a melting peakat a lower temperature (30° C. less) whose endotherm was smaller andmore spread out, meaning that the polymer obtained had smallercrystalline lamellae (platelets). Furthermore, the melting peak duringthe second heating was found in the same position as during the firstheating, which confirms the significant modification of the molecularchain. This observation is evidence of the stability of the material andsignifies a complete polymerization. These measurements show anincontrovertible improvement provided thanks to the composition of thepresent invention.

B. Tensile Mechanical Behaviour

Tensile tests were carried out according to the ISO 527 standard on typeH2 dumb-bell test pieces (working length of 25 mm) at a pull rate(traverse speed) of 25 mm/min. Several test conditions were tested:ambient temperature for non-oven-cured test pieces and temperatures of−40, −10, 20, 50 and 85° C. for oven-cured test pieces.

Overall, the anionic PA-6 polyamides obtained from the composition ofthe invention had a slightly greater rigidity than the hydrolytic PA-6polyamides, as they had a higher molecular weight. On the other hand,the elongation at break was variable depending on the activator content.

This is because, as shown in the appended FIG. 10, the elongationchanged from 5% to 65% when the activator content changed from 0.45% byweight (curve “0.45% ac” in FIG. 10) to 1.1% by weight (curve “1.1% ac”in FIG. 10). This can be explained by an elongation of the chains at alow activator content, thus promoting the bonds between each lamella andthe entanglements of the amorphous phase. Higher elongations could notbe obtained (without additives) due to the great difficulty inpolymerizing with such low activator contents, the lower limit of whichwas around 0.3%.

Since the glass transition temperature was close to 40° C., thebehaviour of the PA-6 varied little from −40° C. to +20° C. On the otherhand, starting from 50° C., the flexibility and the ductility greatlyincreased.

On the mechanical level, the tensile tests represented in FIG. 11 veryclearly show the effect of the additive in the composition of thepresent invention, in particular starting from an addition of 10% ofadditive (curve “10% ad” in FIG. 11). The Young's modulus was divided by3 (from 2200 MPa to 700 MPa) when moving from 0% (curve “0% ad” in FIG.11) to 15% of additive (curve “15% ad” in FIG. 11) which brings it to arigidity comparable to that of a high density polyethylene (curve p).

The elongation at break was itself also considerably increased: changingfrom 10 to 300% elongation per 0 and 15% of additive 6 respectively. Thedamage was of ductile type characterized by a very clearly markednecking. The fracture took place after the necking phase. Similarly, thestress at the yield point was decreased by around two. Thesemeasurements show an incontrovertible mechanical improvement providedthanks to the composition of the present invention.

C. Gaseous Hydrogen Barrier Performance

Measurements of permeability to hydrogen (27° C., 50 bar) were carriedout and collated in graph form in appended FIG. 12.

The first measurements on the reactive routes showed a betterimpermeability of the reactive route PA-6 polyamides than the moltenroute PA-6 polyamides of the prior art (factor of around 2) (not shown).

The smaller lamellae (platelets) and the shorter distance between thelamellae were the reasons for the increase in the tortuosity of thematerial.

Tests carried out at 7×10⁷ Pa (700 bar) on samples confirmed the goodperformance of the PA-6 polyamides obtained by reactive rotomouldingfrom the composition of the present invention, since the permeabilitycoefficient (Pe) was around 4×10⁻¹⁷ mol/(mPas), i.e. a coefficientlower, by a factor of around 5 to 10, than that which is required by thecurrent draft standards (ISO TC 197 and EIHP II) in terms of allowableleakage levels for tanks (1 cm³/l/h).

Tests on a complete tank (200×10⁵ Pa (200 bar), 27° C., hydrogen)confirmed the results obtained with these samples.

Example 8 Manufacture of a Type IV Tank

The tank (1) manufactured is represented in the appended FIG. 14. Inthis example, the envelope (E) manufactured in the Example 3, equippedwith its socket (4) was equipped with a reinforcing structure (6). Forthis, carbon fibres previously impregnated with non-crosslinked epoxyresin were wound around the envelope held by the socket (the linerserved as a mandrel) according to one of the processes described indocuments [4], [5], [24] or [25].

A few layers of glass fibres impregnated with uncrosslinked epoxy resinwere then wound like for the carbon fibres. The spooled tank was thenplaced in a rotating oven in order to cure the epoxy resin.

A protective shell (8) could then be placed around the filament windingas shown in cross section in FIG. 14. A valve/regulator may be screwedinto the tank, in the socket (not shown).

A type IV tank was thus obtained. This tank had the aforementionedsealing specifications.

Example 9 Post-Treatment of a Liner Obtained According to the Process ofthe Invention

An envelope manufactured according to the process of the presentinvention, for example according to the procedure from Example 2, couldbe subjected to a post-treatment such as those cited in the abovedetailed description of the invention, in order to improve its sealingproperties and also its chemical properties of the inner and/or outersurface.

Examples of post-treatments that can be applied to the liner aredescribed in documents [26] and [27] from the appended list ofreferences.

Example 10 Manufacture of a Hydraulic Accumulator

The hydraulic accumulator (10) manufactured in this example isrepresented in the appended FIG. 17.

In this example, a type IV tank was manufactured as in Example 8 exceptthat it was equipped with two sockets (14 and 16) which made it possibleto install the connector of the accumulator (as in Example 4 above).

The body (C) of the accumulator was therefore composed of said type IVtank equipped with its two sockets (14 and 16).

For the assembly of the accumulator, the procedure as indicated indocument [37] from the appended list of references was followed. Aflexible internal pocket (18) made of rubber was placed in the type IVtank, and the first connector (20), to which the pocket was connected ina leaktight manner, at the socket (14), was installed. An anti-extrusionpellet (22) was attached to the pocket to prevent it from exiting thetank via extrusion through the second connector (24) during the use ofthe accumulator. In this example, the pellet was made of vulcanizedplastic.

The connector (20) comprised a supply valve through which it waspossible to introduce a volume V₀ of inert gas into the flexible pocket(18) at a pressure P₀ suitable for use of the accumulator. The pocketcould thus be “inflated” in the tank.

The second connector (24) was then positioned at the socket (16) so asto be able to introduce a fluid at a pressure P₁ into the tank, aroundthe flexible pocket. This connector was such that it could be connectedto a fluid circulation pipe.

Thus, when the accumulator was connected to the fluid circulation pipe,the fluid penetrated into the accumulator and the pressure P₁ of thefluid in the pipe was therefore present in the accumulator around theflexible pocket (18). When the pressure P₁ of the circuit exceeded theinflation pressure P₁ of the flexible pocket of the accumulator, saidpocket contracted and the gas compressed, reducing its volume to V₁. Byfurther increasing the pressure to P₂ (P₂ being greater than P₁), thevolume of gas in the flexible envelope was further compressed andchanged to a volume V₂. The accumulator then had a pressurized fluidvolume of V=V₁−V₂.

A functional hydraulic accumulator was thus obtained. This accumulatorhad the sealing specifications cited in the examples above. Thisaccumulator had a capacity of 3.5 litres and an operating pressure of2.5×10⁷ Pa (250 bar). The maximum pressure (P₂)/pressure at rest (P₀)ratio was 4:1 in this example. It could of course be different in othercases.

A protective shell could of course be placed around the filament windingas in Example 8 and as shown in cross section in FIG. 14 (not shown inFIG. 17).

What is claimed is:
 1. A composition comprising in % by weight relativeto the total weight of the composition: from 70 to 90% of anε-caprolactam monomer of formula (I); from 0.1 to 1% of an ε-caprolactamactivator of formula (II), in which R is selected from the groupconsisting of C_(n)H_(2n+2), n being an integer chosen from 1 to 10;—OH; —OC_(n)H_(2n+2), n being an integer chosen from 1 to 10; and —NHR′where R′ is either C_(n)H_(2n+2), n being an integer chosen from 1 to10, or an amine functional group; from 2 to 6% of an ε-caprolactamcatalyst of formula (III), in which X is chosen from the groupconsisting of MgBr, MgI, Li and Na; and from 10 to 20% of anε-caprolactone additive of formula (IV); wherein the formulae (I), (II),(III) and (IV) are the following:


2. The composition according to claim 1, comprising in % by weight: from80 to 81% of said ε-caprolactam monomer; from 0.3 to 0.5% of saidε-caprolactam activator; from 3.5 to 4.5% of said ε-caprolactamcatalyst; and from 14 to 16% of said ε-caprolactone additive.
 3. Thecomposition according to claim 1, comprising in % by weight: 80.6% ofsaid ε-caprolactam monomer; 0.4% of said ε-caprolactam activator; 4% ofsaid ε-caprolactam catalyst; and 15% of said ε-caprolactone additive. 4.A process of manufacturing an impermeable element comprising thefollowing steps: (a) providing the composition of claim 1 andintroducing said composition to a rotomould; (b) rotation of therotomould and anionic polymerization of the ε-caprolactam monomer ofsaid composition to polycaprolactam, said composition being heated to apolymerization temperature greater than or equal to the melting point ofthe ε-caprolactam and less than the melting point of saidpolycaprolactam, thereby forming said element by rotomoulding coupledwith polymerization without melting of the polycaprolactam obtained; (c)cooling of the rotomould thereby crystallizing the polycaprolactamobtained; and (d) demoulding the polycaprolactam liner formed in therotomould.
 5. The process of claim 4, wherein, in step (a), at least theε-caprolactam monomer and the ε-caprolactam catalyst of said compositionare purged by means of a dry inert gas.
 6. The process of claim 4,wherein, in step (a), two premixes of said composition are prepared, onecontaining the monomer, the activator and the additive, and the othercontaining the monomer and the catalyst, these two premixes being mixedtogether just before, during their introduction, or into the rotomouldto form said composition.
 7. The process of claim 4, wherein, in step(a), the composition is additionally preheated to a preheatingtemperature greater than or equal to the melting point of said monomerand less than said polymerization temperature, before or after theintroduction step (b) so as to melt the composition and homogenize it.8. The process of claim 4, wherein the rotomould is purged by means of adry inert gas during the implementation of step (c).
 9. The process ofclaim 4, wherein the rotomould is rotated about two axes, so that thepolymerization takes place over an entire internal surface of therotomould provided to form the element and conform to the internalsurface.
 10. The process of claim 4, wherein polymerizing theε-caprolactam monomer to polycaprolactam in the rotomould is carried outat a polymerization temperature of between 150° C. to 200° C.
 11. Theprocess of claim 4, wherein steps (a), (b), and (c) are repeated to forman impermeable element having several layers of polycaprolactam that areidentical or different, in thickness and/or in composition.
 12. Theprocess of claim 4, wherein a tank socket is attached to the inside ofthe rotomould before carrying out step (c) whereby the tank socket isincorporated into the impermeable element during the polymerization. 13.The process of claim 4, wherein the impermeable element manufacturedthereby consists of an impermeable envelope.
 14. The process of claim13, wherein the impermeable envelope manufactured thereby is impermeableto pressurized fluids.
 15. The process of claim 13, wherein theimpermeable envelope manufactured thereby is impermeable to gases of acomposite type IV tank.
 16. A hydraulic accumulator comprising animpermeable envelope obtainable by implementation of the processaccording to claim
 4. 17. A process for manufacturing an impermeableelement comprising polymerizing and moulding the composition of claim 1.18. The process of claim 17, wherein the impermeable element is animpermeable envelope.